44 ILLINOIS BIOLOGICAL MONOGRAPHS [214 



nant of the submentum. A similarly situated area found in Myeetobia 

 has been homologized by some writers with that of Rhyphus. This 

 interpretation is undoubtedly incorrect, since this area in Myeetobia 

 (Fig. 90) gives rise to chitinized projections at its ventro-lateral angles 

 and these in turn connect with the maxillary palpi and the galeae. 

 Furthermore, the relationship which this piece bears to the proximal 

 end of the theca (the) would tend to disprove such an interpretation. 

 This piece in Myeetobia is undoubtedly a specialization of the maxillae 

 similar to the modifications found in Bibiocephala (Fig. 83) and Rhab- 

 dophaga (Fig. 86). In aU genera where the postgenae have grown 

 together on the meson the submentum and mentum have been elimi- 

 nated, unless one regards the area between the ventral margin of the 

 head and the theca as derived from these areas. This area, as already 

 described for the Cyclorrhapha, is very extensive and forms the caudal 

 portion of the basiproboscis (bpr). 



The proximal portion of the ligula or mediproboscis (mpr) of the 

 typical labium is largely membranous, but it has on its caudal aspect 

 a distinctly chitinized area, the theca (the), which has a distinct furrow 

 on its meson. The shape, size, and degree of chitinization of the theca 

 vary greatly, as can be seen in Bibio (Fig. 364), Trichocera (Fig. 

 365), Rhyphus (Fig. 374), Promachus (Fig. 376), Tabanus (Fig. 391), 

 Chyromya (Fig. 411), Conops (Fig. 420), Rhamphomyia (Fig. 424), and 

 Musca (Fig. 466). There is a distinct furrow or thickening on the 

 meson of the majority of the Nematocera and the Brachycera, and rem- 

 nants of these thickenings occur also among the Cyclorrhapha. In some 

 of the Diptera the structural condition of the meson has a marked influ- 

 ence on the shape of the dorsal and ventral margins of the theca. The 

 cephalic aspect of the proximal portion of the ligula of a typical labium 

 is concave and membranous and connects with the proximal part of the 

 lance-like portion of the hypopharynx. In the Nematocera the cephalic 

 aspect resembles the typical labium, and in the Brachycera and in a 

 majority of the Cyclorrhapha it has a distinctly chitinized groove. This 

 is well illustrated by Tabanus (Fig. 392), Eristalis (Fig. 441), and a 

 majority of the Calyptratae. The degree of chitinization varies con- 

 siderably, and in some forms heavy, chitinized, cord-like pieces extend 

 along the sides of the groove from the glossae to the proximal end of 

 the labium. 



The distiproboscis of the typical labium is composed of two large 

 independent, highly membranous, bulb-like paraglossae (pgl), usually 

 called oral lobes or labeUae, and two small membranous glossae (gl). 

 Each paraglossa has on its lateral and caudal aspects a Y-shaped chi- 

 tinized support which has been commonly called the furca. For con- 



